In respect of material for lens, after elapse of the most prevailing age of glass, resin lenses which are transparent and excellent in weather resistance have been developed with development of chemical synthesis and acrylic resin, polycarbonate resin, ADC resin and the like have been in use. However, strict physical properties are required for such lenses over a variety of items of refractive index, aberration, polishing performance, impact resistance, weather resistance, scratch resistance and so on. Above all, in respect of refractive index and aberration, development of excellent resin where the both items are balanced has been positively carried out, resin having a function comparable to glass having refractive index of 1.6 and Abbe's number of 35 or more has been on the market and preferably used particularly for plastic lens for spectacles. Further, in respect of plastic lens for spectacle, excellency in dyeing performance is one of significant factors in pursuit of fashionable performance.
Injection molding process and cast forming process are general as methods of producing lens, particularly plastic lens for spectacles, thermoplastic resin is produced mainly by the injection molding process and thermosetting resin is produced by the cast forming process. Although superiority or inferiority of these production methods is not generally evaluated, there is a difference therebetween at the start point of producing lens in which thermoplastic resin is in a high molecule state and thermosetting resin is in low molecule or so-called monomer state. Further, both of them are injected into a sealed lens forming space. In view of only the production process, the injection molding process is a labor saving process and the cast forming process is labor intensive.
Meanwhile, when attention is paid to refractive index that is one of features of lens, the refractive index of acrylic resin mentioned above is 1.49, that of polycarbonate resin is 1.58, that of ADC resin is 1.50, that of urethane is 1.57 and that of sulfur-containing urethane is 1.67. As methods of forming these resin, the injection molding process is used with respect to acrylic resin and polycarbonate resin and the cast forming process is used for ADC resin and sulfur-containing urethane.
When lens having high optical function can be produced by a process with raw material of high molecular material such as by injection molding, it considerably contributes to promotion of productivity of lens, however, thermosetting resin such as ADC resin and sulfur-containing urethane described above, is provided with a steric crosslinked structure and cannot be made plastic and therefore, the injection molding has not been able to be carried out. Hence, the inventors previously developed optical thermoplastic thiourethane-urethane copolymer containing both of thiourethane bond and urethane bond as a thermoplastic copolymer for optical use and disclosed them in U.S. patent application Ser. No. 08/548,806 and PCT International Application No. PCT/JP96/03710. A description will be given of its outline as follows.
The production process of the thermoplastic copolymer for optical use is characterized in including a first step and a second step. In the first step, diisocyanate and/or diisothiocyanate compound is reacted with thiol compound having two groups reactive with isocyanate group, at least one of which is SH group, to form thiourethane prepolymer having weight-average molecular weight of 1,000 to 30,000. In the second step, thiourethane polymer obtained in the first step and diisocyanate compound are reacted with at least one urethane forming monomer selected from hydroxyl-containing monomers having two groups reactive with isocyanate group at least one of which is OH group. The copolymer obtained by this process is provided with the refractive index of 1.59 or more and Abbe's number of 34 or more, the refractive index and Abbe's number are excellently balanced and various properties required for an optical product such as lens or the like are provided.
Further, although the copolymer can continuously be produced by a reactive extruder, the copolymer is liable to be oxidized since temperature of reaction environment is 140.degree. C. or higher and accordingly, it is preferable to maintain a process of charging raw material and inside of the extruder in an atmosphere of an inert gas such as nitrogen or the like. Further, according to the second step, temperature reaches 180.degree. C. or higher and the environment is in a state where extruded resin is extremely liable to be oxidized and accordingly, it is preferable to place the resin similarly in an inert gas atmosphere until temperature of the copolymer reaches the glass transition temperature of 116.degree. C.
Although lens having excellent optical properties can be obtained from a high molecular material by the development of the thermoplastic copolymer, generally, in the case where lens is produced by the injection molding process, a problem is found that optical nonuniformity is liable to be caused by occurrence of flow line or the like and it becomes apparent that the excellent optical properties of the copolymer cannot be sufficiently utilized by the injection molding process.
The present invention has been carried out in view of the above-described drawbacks and it is an object of the present invention to provide a method of continuously producing a lens which is excellent in various optical properties and which has no nonuniformity in the optical properties over a total of the lens by using optical thermoplastic thiourethane-urethane copolymer containing both of thiourethane bond and urethane bone and a lens produced by the method.